+ Both are Awesome action shounen animes.
+ Both share light-hearted humour and comedic aspects of its characters.
+ Both series share similar main protagonist to one another.
(despite age difference)
+ Both shows have very epic fight scenes.
+Both shows have a very powerful antagonist that must be defeated to save a lot people.
Both anime contain themes of friendship, loyalty and trust although these are more prevalent in Fairy Tail. Besides sharing similar keynote themes both anime have characters who undergo both personal and physical transformations in order to meet/face new adversaries and overcome various challenges. Both contain a fair amount of action with a good balance of comedy whilst still been serious when the situation/story calls for it. If you like one you will probably like the other.